Going thru the ranks, we were always told to be ready not only for the current test material but some stuff from the next test as well.
I would also emphasize the opposite! So often we get caught up with the test itself, and focus on that particular test's requirements. We often like to throw a wrench in the cog and make people perform techniques from previous tests. This ensures retention, sets a standard, and messes with people's confidence (its important to put people on the spot and see how they react). Often, the teacher will choose some super abscure or basic concept, like a basic roll or tai sabaki type movement (we do kihon dosa...a little differenct, but idea is the same). The teacher wants to see several things by doing this; do you remember this stuff, are you constantly practicing, can you actually teach it, and has techniques you haven't been required to 'display' on a test in years been progressing along with the newer, fancier, techqniues required at your level. But, in practice, and in the kyu levels my dojo typically adds a thing or two from higher ranks that are not in the approved national and international curriculum. But it will always be something students should be prepared for and have seen before.
again, sorry about spelling....spell checker no workie.